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United States Patent |
5,607,566
|
Brown
,   et al.
|
March 4, 1997
|
Batch deposition of polymeric ion sensor membranes
Abstract
Screen printing technology is employed in the batch fabrication of the
contacts and polymeric membranes of solid-state ion-selective sensors. The
process achieves high yield with very reproducible results. Moreover,
membrane thickness can easily be predetermined, as it is directly related
to the thickness of the screen or stencil. The process of the present
invention is compatible with many integrated circuit manufacturing
technologies, including CMOS fabrication. Advantageous polymeric membrane
paste compositions include a polyurethane/hydroxylated poly(vinyl
chloride) compound and a silicone-based compound in appropriate solvent
systems to provide screen-printable pastes of the appropriate viscosity
and thixotropy.
Inventors:
|
Brown; Richard B. (Ann Arbor, MI);
Cha; Guen-Sig (Seoul, KR);
Goldberg; Howard D. (Ann Arbor, MI)
|
Assignee:
|
The Board of Regents of the University of Michigan (Ann Arbor, MI)
|
Appl. No.:
|
196105 |
Filed:
|
October 3, 1994 |
PCT Filed:
|
August 20, 1992
|
PCT NO:
|
PCT/US92/07037
|
371 Date:
|
October 3, 1994
|
102(e) Date:
|
October 3, 1994
|
Current U.S. Class: |
257/414; 204/403.06; 204/413; 204/416; 252/62.2; 252/500; 427/97.2; 427/97.5; 427/98.3; 427/282 |
Intern'l Class: |
G01N 027/26 |
Field of Search: |
204/403,416,418,413
252/62.2,500
427/282
|
References Cited
U.S. Patent Documents
4454007 | Jun., 1984 | Pace | 204/418.
|
4670490 | Jun., 1987 | Yoshida et al. | 524/115.
|
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Rohm & Monsanto
Parent Case Text
RELATIONSHIP TO OTHER APPLICATIONS
This is a 371 of PCT/US Ser. No. 92/07037 filed Aug. 20, 1992, and a
continuation-in-part patent application of U.S. Ser. No. 748,742 filed on
Aug. 20, 1991 now abandoned as a continuation-in-part of U.S. Ser. Nos.
370,897, filed Jun. 23, 1989, now abandoned; 517,636, filed May 2, 1990;
and 517,651, also filed May 2, 1990, now abandoned. U.S. Ser. No. 517,636
issued as U.S. Pat. No. 5,102,526 on Apr. 7, 1992. The remainder of the
aforementioned patent applications are pending at the time of filing the
present application, were filed in the names of Richard B. Brown and
Geun-Sig Cha, both of whom are inventors herein, and all such applications
are assigned to the same assignee as herein. The disclosures of all of
said aforementioned patent applications are incorporated herein by
reference.
Claims
What is claimed is:
1. A method of batch fabricating ion-selective sensors, the method
comprising the steps of:
installing a mask on a semiconductor substrate, the mask having at least
one aperture therethrough having a set configuration which corresponds to
a chosen membrane configuration;
applying a polymeric membrane paste to the mask, the polymeric membrane
being in the form of a mixture of a polyurethane polymer, hydroxylated
poly(vinyl chloride) copolymer, ionophore, and a plasticizer, as polymeric
membrane components dissolved completely in a second solvent after a first
solvent has been removed from the polymeric membrane paste;
drawing a squeegee across said mask wherein the polymeric membrane paste is
urged into the aperture of the mask and into communication with the
semiconductor substrate; and
curing the deposited polymeric membrane paste to form a polymeric membrane
having a set ion-selective characteristic.
2. The method of claim 1 wherein said mask is formed of a metallic
material.
3. The method of claim 2 wherein the mask is formed of a stainless steel
mesh and there is further provided the step of coating the stainless steel
mesh with a photoreactive emulsion.
4. The method of claim 2 wherein the mask is formed of a metal-foil
stencil.
5. A method of batch fabricating ion-selective sensors, the method
comprising the steps of:
installing a metallic mask on a semiconductor substrate on which are
simultaneously formed a plurality of the ion-selective sensors, the mask
being patterned to have a plurality of apertures therethrough, each having
a set configuration which corresponds to a chosen membrane configuration,
and each being located as to be associated with one of the ion-selective
sensors;
applying a polymeric membrane paste to the mask, the polymeric membrane
being in the form of a mixture of a polyurethane polymer, hydroxylated
poly(vinyl chloride) copolymer, ionophore, and a plasticizer, as polymeric
membrane components dissolved completely in a second solvent after a first
solvent has been removed from the polymeric membrane paste;
urging the polymeric membrane paste into the pattern of apertures of the
mask and into communication with the semiconductor substrate; and
curing the deposited polymeric membrane paste to form a polymeric membrane
having a set ion-selective characteristic.
6. The method of claim 5 wherein the step of urging comprises drawing a
squeegee across the mask at a set distance from the mask with a set
squeegee speed, squeegee shape, squeegee angle, squeegee pressure, and
squeegee push-in quantity.
7. A solid-state ion-selective sensor formed by the process of:
installing a metallic mask on a semiconductor substrate on which are
simultaneously formed a plurality of ion-selective sensors, the mask
having a pattern formed of a plurality of apertures therethrough, each
such aperture having a set configuration which corresponds to a chosen
membrane configuration for a respectively associated one of the
ion-selective sensors;
applying a polymeric membrane paste to the mask, the polymeric membrane
being in the form of a mixture of a polyurethane polymer, hydroxylated
poly(vinyl chloride) copolymer, ionophore, and a plasticizer, as polymeric
membrane components dissolved completely in a second solvent after a first
solvent has been removed from the polymeric membrane paste;
urging the polymeric membrane paste into the pattern of apertures of the
mask and into communication with the semiconductor substrate; and
curing the deposited polymeric membrane paste to form a polymeric membrane
having a set ion-selective characteristic.
8. The solid state ion-selective sensor of claim 7 wherein the step of
urging comprises drawing a squeegee across the mask at a set distance from
the mask with a set squeegee speed, squeegee shape, squeegee angle,
squeegee pressure, and squeegee push-in quantity.
9. A process for forming a polymeric membrane paste, the process comprising
the steps of:
mixing polyurethane, hydroxylated poly(vinyl chloride) copolymer ionophore,
and a plasticizer, as polymeric membrane components, in a first solvent
have a first boiling point in order to dissolve completely the components;
adding a second solvent having a second boiling point higher than the first
boiling point; and
removing the first solvent.
10. The process of claim 9 further comprising the step of adding an
adhesion promotor after the step of removing the first solvent.
11. The process of claim 9 wherein the step of mixing further includes
adding an ionophore.
12. A process for forming a polymeric membrane paste, the process
comprising the steps of:
mixing silicone rubber and a lipophilic additive in a solvent in order to
completely dissolve the components; and
evaporating the solvent.
13. The process of claim 12 wherein the polymeric membrane components
comprise silicone rubber and a lipophilic additive.
14. The process of claim 12 wherein the step of mixing further includes
adding an ionophore.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to systems for producing ion sensors, and
more particularly, to a system which produces ion sensors which employ
ion-responsive membranes formed of polymeric materials using screen
printing procedures to achieve high reproducibility.
In order for an ion sensor to be commercially acceptable and successful, it
must be possessed of qualities beyond electrochemical performance. In
order for a sensor to be cost effective, it must be reproducible using
mass production systems. Moreover, there must be common electrochemical
response characteristics within the members of a batch fabricated group.
If the sensors are not all substantially identical, they will each be
characterized by different lifetimes and response characteristics,
creating difficulties in the field, not the least of which is the added
cost associated with recalibration of equipment whenever the sensor is
changed. In general, polymeric membranes are in common use as transducers
in solid-state chemical sensors, particularly because such membranes have
high selectivity to the ion of interest and can be made selective to a
wide range of ions using one or many readily available ionophores. One
known technique for forming the membranes is solvent casting; a technique
which originated with ion-selective electrode technology. In this
approach, the membranes are cast by dissolving their components in an
organic solvent, hand depositing the solution onto the sensor sites, and
allowing the solvent to be removed by evaporation. In addition to being a
rather tedious operation, particularly in view of the small size of the
sensors, this production method yields very high losses. The thickness and
shape of the membrane cannot be controlled, resulting in an unacceptable
lack of sensor reproducibility.
These problems and concerns have been addressed in the prior art, but
adequate solutions have not been found. For example, one known approach
involves the use of a blank membrane solution to form a coating which
conforms to the sensor. The membrane coating is then selectively doped
over the multiple sensor sites. This technique suffers from the
disadvantage that considerable hand work must be performed under a
microscope. In addition, the ionophore will diffuse laterally over time.
Moreover, electrical interference in the form of cross-talk through the
membrane would limit the geometries and spacings between input pads to
dimensions which are unacceptable for multisensing devices.
A further known system employs a lift-off method for patterning
permselective membranes. This system requires the patterning of the
silicon wafer with a positive photoresist, which results in the
photoresist being removed in the areas where the-membrane is to be
deposited. The dissolved membrane solution is spin coated onto the wafer
and allowed to dry. The wafer is immersed in an ultrasonic solvent bath
which removes the membrane-coated photoresist regions, resulting in a
precisely patterned wafer. This method suffers from the disadvantage of
exposing the membranes to organic lift-off solvents which may alter the
electrochemical characteristics of the membranes. In addition, the range
of thicknesses is limited to those which can be realized in photoresist.
Moreover, in multisensing devices, cross-contamination in the ultrasonic
lift-off bath can be a problem.
There is not currently available any suitable system for batch fabrication
of solid-state ion-selective sensors which employ polymeric membranes.
Such membranes can be formed of a variety of polymeric materials, such as
poly(vinyl chloride), polyurethane, and silicone.
The use of polyurethane and silicone in the ion-selective membranes of
chemical sensors is described in the parent applications enumerated
hereinabove. These applications are all incorporated herein by reference.
It is, therefore, an object of this invention to provide a simple and
economical system for batch fabrication of solid-state ion-selective
sensors.
It is another object of this invention to provide a system for mass
producing solid-state ion-selective sensors which employ ion-selective
membranes formed of polymeric materials.
It is also an object of this invention to provide a system for batch
fabrication of solid-state ion-selective sensors which results in high
yield and with high uniformity between the respective sensors.
It is a further object of this invention to provide a substance-sensitive
membrane system for a solid-state sensor which can be applied to a
plurality of solid-state devices simultaneously using conventional
techniques.
It is additionally an object of this invention to provide a
substance-sensitive polymeric membrane system for a solid-state sensor
which can be applied to a multiplicity of solid-state devices
simultaneously using conventional integrated circuit manufacturing
techniques.
It is yet a further object of this invention to provide a
substance-sensitive membrane for use with a solid-state sensor which does
not require a structural layer associated therewith to maintain
communication between the membrane and a solid-state substrate.
SUMMARY OF THE INVENTION
The foregoing and other objects are achieved by this invention which
provides a method of batch fabricating ion-selective sensors. In
accordance with the invention, the method comprises the steps of:
installing a mask on a semiconductor substrate, said mask having at least
one aperture therethrough having a predetermined configuration which
corresponds with a desired membrane configuration;
applying a polymeric membrane paste to said mask; and
drawing a squeegee across said mask whereby said polymeric membrane paste
is urged into said aperture of said mask and into communication with said
semiconductor substrate.
In one embodiment of the invention, the mask is formed of a metallic
material, which may be a stainless steel mesh. Further, in accordance with
the method aspect of the invention, the stainless steel mesh is coated
with a photoreactive emulsion.
In a further embodiment, the mask is formed of a metal foil stencil. The
membrane which ultimately is produced has a thickness which corresponds to
that of the mask. In practical embodiments of the invention, such a
thickness may be between 25/.mu.m and 250 .mu.m.
As previously stated, the membrane is formed of a polymeric membrane paste.
Such a paste may be formed of a polyurethane with an effective portion of
an hydroxylated poly(vinyl chloride) copolymer therein as described in
U.S. Ser. No. 517,651; a polyimide-based compound as described in U.S.
Ser. No. 746,134; a silicone-based compound, such as silanol-terminated
polydimethylsiloxane with the resistance-reducing additive, CN-derivatized
silicone rubber described in U.S. Pat. No. 5,102,526; or any other
suitable polymeric material.
In a further method step, the screen printed polymeric membrane material is
cured after the mask is removed.
In accordance with a product-by-process aspect of the invention, a
solid-state ion-selective sensor formed by the process of:
installing a metallic mask on a semiconductor substrate on which are
simultaneously formed a plurality of ion-selective sensors, the mask
having a pattern formed of a plurality of apertures therethrough, each
such aperture having a predetermined configuration which corresponds with
a desired membrane configuration for a respectively associated one of the
ion-selective sensors;
applying a polymeric membrane paste to the mask; and
urging the polymeric membrane paste into the pattern of apertures of the
mask and into communication with the semiconductor substrate.
Certain embodiments of the invention include the further step of drawing a
squeegee across the mask whereby the polymeric membrane paste is urged
into the apertures of the mask and into communication with the
semiconductor substrate. In further steps, the mask is removed and the
membrane paste is cured.
BRIEF DESCRIPTION OF THE DRAWING
Comprehension of the invention is facilitated by reading the following
detailed description, in conjunction with the annexed drawing, in which:
FIG. 1 is a simplified schematic side view which is useful in describing a
screen printing process for forming an ion selective membrane;
FIG. 2 is a simplified schematic representation of a solid-state
microelectrode constructed in accordance with the invention;
FIG. 3 is a graphical representation of the ammonium ion response of screen
printed polyurethane-based membranes of the present invention plotted as a
function of electrode response in mV against the log of the ammonium ion
concentration in moles over the concentration range 10.sup.-3 to
10.sup.-1.5 M;
FIG. 4 is a graphical representation of the ammonium ion response of screen
printed silicone rubber-based membranes of the present invention plotted
as a function of electrode response in mV against the log of the ammonium
ion concentration in moles over the concentration range 10.sup.-3 to
10.sup.-1.5 M; and
FIG. 5 is a schematic cross-section of a single electrode site on a
multisensor.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a simplified schematic representation of a silicon wafer upon
which is being deposited a polymeric membrane 11. In the practice of the
invention, polymeric membrane 11 is ion-selective, as described in the
co-pending patent applications which have been incorporated herein.
In the practice of the process of the invention, a mask 12 is installed on
silicon wafer 10. The mask has an aperture 13 in which is shown to be
deposited the polymeric membrane material. Mask 12 may be formed of a
stainless steel mesh coated with a photoreactive emulsion (not shown).
Alternatively, the mask may be formed as a metal-foil stencil. In another
embodiment, the mask is patterned with the desired features for membrane
printing.
A screen printer (not shown) evenly applies the membrane paste, the excess
of which is indicated as paste 15, and rubs the paste with a squeegee
member 16 which pushes the paste through aperture 13 and onto silicon
wafer 10 which functions as a substrate. Squeegee member 16 is, in this
embodiment, configured in the shape of a diamond and is moved in the
direction of the arrow shown in the figure.
As can be seen from this figure, the thickness of polymeric membrane 11 is
responsive to the thickness of mask 12. In practical embodiments of the
invention, the mask can have a thickness of approximately between 25
microns and 250 microns. A modem, optical-aligned screen printer, such as
model LS-15TV which is commercially available from the New Long Seimitsu
Kogyo Company, allows alignment and reproducibility to approximately .+-.5
microns.
The print quality of the deposited material is a function of mask clearance
from the substrate, squeegee speed, squeegee shape, squeegee angle,
squeegee pressure, and squeegee push-in quantity. Edge quality of the
pattern is determined by squeegee shape and mask clearance from the
substrate. Pattern flow-out and thickness is determined primarily by
squeegee speed, pressure, and push-in quantity. If squeegee speed is too
fast, or is accomplished without enough pressure or push-in quantity, the
pattern may not be completely filled with paste, and the deposited
material may have peaks, rather than a smooth profile. If the squeegee
speed is too slow, or the pressure and push-in quantifies too great, the
pattern flow-out will increase and thickness will be decreased due the
scavenging effects of the squeegee.
After the membrane paste is applied to the silicon wafer, it must be cured,
illustratively, by drying in the air or in an oven at elevated
temperatures. Curing conditions are within the skill of a person of
ordinary skill in the art. However, we have found that curing should be
controlled to avoid evaporation of the membrane components, specifically a
plasticizer, if included.
The screen printing method of fabricating solid-state ion-selective sensors
of the present invention imposes rheological constraints upon the membrane
material. Solvents and additives are used to form the membrane paste, such
as paste 15, having an appropriate viscosity and thixotropy to achieve
good pattern definition. Viscosity must be adjusted to achieve the
appropriate resistance to flow from squeegee motion and thixotropy must be
adjusted for appropriate resistance to secondary flow after the mask is
removed from the substrate.
For example, anhydrous tetrahydrofuran (THF), which is typically used for
solvent cast membranes, is an unacceptable solvent for a screen printing
paste due to its high evaporation rate which causes the viscosity of the
paste to change rapidly, resulting in clogging of the mask. Other commonly
used solvents with lower evaporation rates (i.e., higher boiling points),
such as N,N-dimethylacetamide (DMA) and cyclohexanone, or combinations of
these solvents and THF, have been tried, but yielded less than
satisfactory results. However, in certain embodiments, we have found that
THF in the solvent system advantageously facilitates dissolution of the
membrane components. THF can be removed from the membrane paste prior to
use, for example, by permitting evaporation in a vacuum desiccator.
Use of too little solvent results in a tacky, stringy paste that gels on
the mask whereas use of too much solvent results in thinner membranes with
poorer pattern definition. The screen printing process itself exacerbates
the problem as it continuously spreads a thin layer of paste onto the
screen mask thereby increasing the surface area of the membrane paste
exposed to air and increasing the solvent evaporation rate.
Another unsuccessful technique used to increase viscosity was the addition
of silica powder to the paste composition. The addition of silica powder
resulted in poor printability of the membrane paste and membranes with
pinholes and other defects. Described hereinbelow are several specific
illustrative examples of membrane components and solvent systems used for
screen printing good quality ion-selective membranes in accordance with
the present invention.
EXAMPLE 1
Polyurethane-based Membrane Paste
26.4 wt. % polyurethane (PU; SG-80A, Tecoflex, Thermedics, Inc., Woburn,
Mass.)
6.6 wt. % hydroxylated PVC (PVC/Ac/Hydroxy Propyl Acrylate, 80/5/15 wt. %)
Scientific Polymer Products, Ontario, N.Y.)
66 wt. % plasticizer (bis(2-ethylhexyl)adipate, Fluka, Ronkonkoma, N.Y.)
1 wt. % ionophore (e.g., valinomycin or nonactin)
In one illustrative example, the membrane components were completely
dissolved in THF. The high boiling point solvent, 1-methyl-2-pyrrolidinone
was added to the solution and thoroughly mixed. Then, the THF was removed
in a vacuum desiccator. A membrane paste with good viscosity and screen
printability was achieved.
In a preferred embodiment of the invention, PI-Thinner, a proprietary
mixture of various high-boiling point solvents, available from Epoxy
Technology, Billerica, Mass. was used in the solvent system. In an
illustrative method embodiment, the membrane components were dissolved in
1.2 ml THF. Then, 0.1 ml PI-Thinner were added and allowed to mix
thoroughly. HF was evaporated from the resulting membrane paste.
In some embodiments of the invention, a silanating agent or adhesion
promotor, silicon tetrachloride (SiCl.sub.4), was added to the paste prior
to printing in order to increase membrane adhesion to the semiconductor
surface and to improve the resulting electrode stability. In the preferred
embodiment described immediately hereinabove, SiCl.sub.4 (7 wt. %) was
added just prior to printing.
An illustrative cure cycle for the preferred embodiment consists of a 1
hour convection oven bake (70.degree. C.) to accelerate the PI-Thinner
evaporation process, followed by a room temperature cure for approximately
24 hours.
Of course, other plasticizers, ionophores, or fillers known in the art may
be used in the illustrative membrane compositions set forth herein without
departing from the principles of the invention.
EXAMPLE 2
Silicone Rubber Membrane Paste
A moisture-curable silicone rubber-based formulation comprises:
97.2 wt. % silicone rubber (RTV 3140; Dow Coming, Midland, Mich.)
1.0 wt. % lipophilic additive, e.g., potassium
tetrakis(.rho.-chlorophenyl)borate (Fluka, Ronkonkoma, N.Y.)
1.8 wt. % ionophore
The membrane components were completely dissolved in 1.2 ml THF. The THF
was evaporated and the resulting paste is ready for screen printing. No
adhesion promoting agent, such as SiCl.sub.4, is necessary or desirable in
this composition. The resulting membranes can be cured at room temperature
for 24 hours in the ambient atmosphere to allow the vulcanizing process to
occur.
EXAMPLE 3
Screen-Printed Solid-State Ion-Selective Electrodes
FIG. 2 is a simplified schematic representation of a solid-state
microelectrode 20 which was fabricated using the screen printing system of
the present invention with CMOS-compatible technology. Solid-state
microelectrode 20 is shown to have a silicon substrate 21 with a layer of
silicon dioxide 22 thereon. An aluminum electrode 23 is deposited on the
silicon dioxide layer and a layer of silicon nitride 25 is arranged over
the aluminum electrode and the silicon dioxide layer. A screen printing
process similar to that described hereinabove with respect to FIG. 1 was
employed to produce a silver epoxy contact 26. The epoxy may be of the
type which is commercially available. In addition, a polymeric membrane 27
was also produced using the screen printing process and arranged to
overlie the solid silver epoxy contact. Thus, screen printing technology
is applicable to the fabrication of the contacts and the membranes.
It is highly desirable that the ion-selective membranes, such as the
polyurethane-based membrane described in Example 1, be of a type which
adheres well to silicon-based materials, such as silicon nitride layer 25.
Such adhesion reduces the probability that electrolyte shunts will form
behind the membrane, rendering the solid-state microelectrode inoperative.
In a specific illustrative embodiment, the sensor dimensions shown in FIG.
2 are 1.5 cm by 1.0 cm and the silicon nitride via hole it the electrode
site is 600 .mu.m.sup.2. Stainless steel stencil masks (Micro-Screen,
South Bend, Ind. were used to print silver epoxy electrode contacts
(Epotek H20E; Epoxy Technology, Billeries, Mass.) and the polymer
membranes on solid-state sensors. The silver epoxy contact is ideally 660
.mu.m on a side and 102 .mu.m thick. The silver epoxy contact was
deposited by screen printing Epotek H20E and curing for 15 minutes in a
150.degree. C. oven.
Table 1 shows the screen printing parameters used to print the silver and
polymeric sensor layers in the device of FIG. 2. The polymeric sensor
layers comprise membrane paste as prepared in the preferred embodiment of
Example 1 (PU/PVC/Ac/Al) and Example 2 (silicone rubber). The parameters
may be adjusted from run-to-run to compensate for slight variations in the
rheology of the membrane paste.
TABLE 1
______________________________________
Silver Silicone
Parameter Epoxy PU/(PVC/Ac/Al)
Rubber
______________________________________
mask clearance
0 mm 0 mm 0 mm
squeegee shape
diamond square edge square edge
squeegee speed
100 mm/sec
100 mm/sec 150 mm/sec
squeegee angle
none 60.degree. 60.degree.
squeegee pressure
0.9 kg/cm.sup.2
1.0 kg/cm.sup.2
1.1 kg/cm.sup.2
push-in quantity
0.1 mm 0.1 mm 0 mm
______________________________________
The pattern definition quality of the respective screen printed layers is
summarized in Table 2. Lateral flow-out was determined by an automatic
surface profiler (Sloan DekTak II) and film thickness was determined by a
scanning electron microscope image of a cleaved sample.
TABLE 2
______________________________________
Silver Silicone
Quality Epoxy PU/(PVC/Ac/Al)
Rubber
______________________________________
mask 76 .mu.m 127 .mu.m 127 .mu.m
thickness
(.+-. 12.7 .mu.m)
layer 40 .mu.m 83 .mu.m 127 .mu.m
thickness
lateral 25 .+-. 16 .mu.m
66 .+-. 12 .mu.m
48 .+-. 10 .mu.m
flow-out
______________________________________
EXAMPLE 4
Ammonium Ion Sensing Electrodes
Using the compositions of Examples 1 and 2 as the printing paste,
electrodes were fabricated in accordance with the method and parameters
set forth hereinabove in Example 3. Nonactin (Fluka, Ronkonkoma, N.Y.) was
used as the ionophore in the formulation to create an ammonium ion
sensitive electrode.
Using a sleeve-type double junction Ag/AgCl electrode (Orion, Model 90-02)
as the external reference electrode, calibration curves plots obtained by
taking emf measurements every 10 seconds from additions of standard
solutions of ammonium chloride in 250 ml background electrolyte (0.05
mol/L Tris-HCl, pH 7.2) at room temperature. FIGS. 3 and 4 show the
ammonium ion response of the screen printed ion-selective membranes of the
present invention. Referring to FIG. 3, the average slope for the 3
sensors tested with the PU/PVC/Ac/Al) membrane is 51.4 mV/decade over the
concentration range 10.sup.-3 to 10.sup.-1.5 M. Referring to FIG. 4, the
average slope for the 3 sensors with the silicone rubber membrane is 48.9
mV/decade. The sensors were soaked in Tris-HCl buffer (pH 7.2), at room
temperature, between measurements. The response of the sensors was
measured each day for a 33 day period and found to be quite stable.
EXAMPLE 5
Multisensor Chip
FIG. 5 is a schematic cross-section of a single electrode site on a
multisensor chip 30. More specifically, the circuitry is realized by a 2
.mu.m double-metal double-polysilicon p-well process using
silicon-on-insulator (SOI) wafers. An ion-selective electrode has silver
epoxy contact 31, which connects directly to an SOI transistor 34 of an
operational amplifier buffer below it. Contact 31 is coated with a polymer
membrane 32 which additionally communicates with a silicon nitride layer
33. A first metal layer 36 is coupled to SOI transistor 34. A second metal
layer 37 functions as a ground shield in this embodiment to prevent
long-term encapsulation layer breakdown. The SOI transistor and the first
and second metal layers are arranged in a layer 38 of silicon dioxide,
which is itself deposited on a silicon substrate 39. Thus, in this
embodiment, there is achieved three-dimensional dielectric encapsulation
of all circuit nodes from the test solution. The sensor-specific layers
were post-deposited on the microelectronics using the screen printing
techniques of the present invention.
The screen printing techniques of the present invention can be used to
devise a wide variety of solid-state sensors and actuators. For example,
biosensors can be formed by the screen printing techniques described
herein, by printing asymmetrical (multilayer) membranes or bioreactive
reagents suspended in gels over the sensor sites. Various combinations of
ion- and bio-selective electrode sites could be realized on a monolithic
chip containing appropriate microelectronics.
Although the invention has been described in terms of specific embodiments
and applications, persons skilled in the art can, in light of this
teaching, generate additional embodiments without exceeding the scope or
departing from the spirit of the claimed invention. Accordingly, it is to
be understood that the drawing and description in this disclosure are
proffered to facilitate comprehension of the invention and should not be
construed to limit the scope thereof.
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